BUG - IDEX L2B spin phase coordinates should report bin centers

[aldo9253]

Description of the issue

Problem

L2B correctly assigns events to these circular spin-phase ranges:

Range Correct center
━━━━━━━━━━ ━━━━━━━━━━━━━━━━
315–45° 0°
────────── ────────────────
45–135° 90°
────────── ────────────────
135–225° 180°
────────── ────────────────
225–315° 270°

However, the output spin_phase coordinate contains:

[45, 135, 225, 315]

These values are boundaries of the physical bins, not their centers. Each histogram count is therefore associated with a misleading DEPEND_1 coordinate.

Recommended changes

1. Retain the existing circular binning algorithm.

2. Publish the spin-phase coordinate as:

   [0, 90, 180, 270]

3. Update metadata to explicitly describe the bin ranges and centers.

4. Add tests that verify both event assignment and output coordinates.

Steps to reproduce the issue

No response

Expected vs Actual behavior

No response

Code Snippet (If applicable)

Code

Additional notes, affected areas, and suggested fixes

No response